storage: Add mixed fc_host/scsi_host duplicate adapter source checks

storage: Add mixed fc_host/scsi_host duplicate adapter source checks

https://bugzilla.redhat.com/show_bug.cgi?id=1159180

The virStoragePoolSourceFindDuplicate only checks the incoming definition
against the same type of pool as the def; however, for "scsi_host" and
"fc_host" adapter pools, it's possible that either some pool "scsi_host"
adapter definition is already using the scsi_hostN that the "fc_host"
adapter definition wants to use or some "fc_host" pool adapter definition
is using a vHBA scsi_hostN or parent scsi_hostN that an incoming "scsi_host"
definition is trying to use.

This patch adds the mismatched type checks and adds extraneous comments
to describe what each check is determining.

This patch also modifies the documentation to be describe what scsi_hostN
devices a "scsi_host" source adapter should use and which to avoid. It also
updates the parent definition to specifically call out that for mixed
environments it's better to define which parent to use so that the duplicate
pool checks can be done properly.